A 4000-year debris flow record based on  amphibious investigations of fan delta  activity in Plansee (Austria, Eastern Alps)

Abstract. The frequency of debris flows is hypothesized to have increased in recent decades with enhanced rainstorm activity. Geological evidence to test the relationship between climate and debris flow activity for prehistoric times is scarce due to incomplete sediment records, complex stratigraphy, and insufficient age control, especially in Alpine environments. In lacustrine archives, the link between onshore debris flow processes and the sedimentary record in lakes is poorly investigated. We present an amphibious characterization of alluvial fan deltas and a continuous 4000-year debris flow record from Plansee (Tyrol, Austria), combining light detection and ranging (lidar) data, swath bathymetry, and sediment core analyses. The geomorphic investigation of two fan deltas in different developmental stages revealed an evolutionary pattern of backfilling and new channel formation onshore, together with active subaqueous progradation on a juvenile fan delta, major onshore sediment deposition, and only few, but larger, subaqueous deposits on a mature fan delta. Geomorphic evidence for stacked and braided debris flow lobes, subaquatic landslide deposits, and different types of turbidites in sediment cores facilitated a process-based event identification, i.e. distinguishing between debris-flow-induced or earthquake-induced turbidites throughout the 4000-year sedimentary record. We directly correlate subaqueous lobe-shaped deposits with high backscatter signals to terrestrial debris flow activity of the last century. Moreover, turbidite thickness distribution along a transect of four cores allows us to pinpoint numerous events as being related to debris flow activity on a juvenile fan delta. In the sediment core, debris-flow-induced turbidites feature a more gradual fining upward grain size trend and higher TOC (total organic carbon) and δ13C values compared to earthquake-induced turbidites. The 4000-year event record contains 138 debris-flow-induced turbidites separated into four phases of similar debris flow activity (df phases). df phase 1 (∼2120 to ∼2040 before the common era – BCE) reflects the second-highest observed event frequencies and is interpreted as being a postseismic landscape response. After a long period of long recurrence intervals without any outstanding increases in debris flow activity during df phase 2 (∼2040 BCE to ∼1520 common era – CE), there are slightly increased event frequencies in df phase 3 (∼1520 to ∼1920 CE). df phase 4 (∼1920 to 2018 CE) exhibits a drastic increase in debris flow activity, followed by the overall highest debris flow frequency of the whole record, which is about 7 times higher than during df phase 3. We show that the frequency increase in the debris-flow-induced turbidite record matches a previously postulated increase in debris flow events derived from aerial photography at Plansee in the last century. The triggering of debris flows is more controlled by short, intense precipitation than any other mass movement process, and we demonstrate that lacustrine debris flow records provide a unique inventory of hazard-relevant rainstorm frequencies over decades, centuries, and millennia. The presented increase in debris flow frequency since the start of the 20th century coincides with a twofold enhanced rainstorm activity in the Northern European Alps and, therefore, provides a novel technique for the systematic understanding of non-stationary debris flow frequencies in a changing climate.

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A 4,000 year debris-flow record based on amphibious investigations of fan delta activity in Plansee (Austria, Eastern Alps)

10.5194/esurf-2021-23 ◽

2021 ◽

Author(s):

Carolin Kiefer ◽

Patrick Oswald ◽

Jasper Moernaut ◽

Stefano Claudio Fabbri ◽

Christoph Mayr ◽

...

Keyword(s):

Debris Flow ◽

Debris Flows ◽

Phase 1 ◽

Fold Increase ◽

European Alps ◽

Frequency Increase ◽

Fan Deltas ◽

Flow Frequency ◽

Flow Activity ◽

Common Era

Abstract. The frequency of debris flows is hypothesized to increase in recent decades with enhanced rainstorm activity. Geological evidence to test this tendency for prehistoric times is scarce due to incomplete sediment records, complex stratigraphy, and insufficient age control especially in Alpine environments. In lacustrine archives, the link between onshore debris-flow processes and the depositional record in lake depocentres is poorly investigated. We present an amphibious characterization of alluvial fan deltas and a continuous 4,000 year debris-flow record from Plansee (Tyrol, Austria) combining Light detection and ranging (LiDAR) data, swath bathymetry, and sediment core analyses. The geomorphic investigation of two fan deltas in different developmental stages revealed a sediment delivery ratio of 7.9 % for the juvenile fan and no sediment transport into the lake on the mature fan within a 3-month summer period (May 2019–August 2019). Event deposits were dated and categorized according to their causal mechanism in a transect of four sediment cores. Debris flow-induced turbidites feature a more gradual fining-upward grain-size trend and higher TOC and δ13C values compared to earthquake-induced turbidites. Over the last 4,000 years, the record containing 138 debris flow-induced turbidites reveals four different debris-flow activity phases. Phase 1 (2050–1960 before the common era; BCE) depicts the second highest observed event frequencies. Phase 2 (1960 BCE–1550 common era; CE) shows large recurrence intervals. Phase 3 (1550–1905 CE) displays a gradual increase of event frequency. Phase 4 (1905–2018 CE) exhibits a debris-flow frequency increase between 1908 and 1928 CE, followed by the overall highest debris-flow frequency between 1928 and 1978 CE, and lower debris-flow frequencies since 1978 CE, which still exceed those of phase 1 to 3. Most remarkably, we find a ~7-fold increase of debris-flow frequency compared to the reference period 1700–1900 CE. The triggering of debris flows is more controlled by short intense rainstorms than for any other mass movement process and we demonstrate that lacustrine debris-flow records provide a unique inventory of hazard-relevant rainstorm frequencies over decades, centuries, and millennia. In a calibration period of 7 decades, we can show that the debris flow-induced turbidite record matches with the previously published debris-flow volume increase derived from aerial photography coincident to a pronounced rainstorm frequency increase. Here we show a millennium-scale debris-flow record that documents a ~7-fold increase in debris-flow frequencies in the 20th and 21st century coincident to 2-fold enhanced rainstorm activity in the Northern European Alps and provide a novel basis for systematic non-stationary estimation of future debris-flow frequencies in a changing climate.

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Debris-flow activity in abandoned channels of the Manival torrent reconstructed with LiDAR and tree-ring data

Natural Hazards and Earth System Science ◽

10.5194/nhess-11-1247-2011 ◽

2011 ◽

Vol 11 (5) ◽

pp. 1247-1257 ◽

Cited By ~ 22

Author(s):

J. Lopez Saez ◽

C. Corona ◽

M. Stoffel ◽

A. Gotteland ◽

F. Berger ◽

...

Keyword(s):

Debris Flow ◽

Tree Ring ◽

Debris Flows ◽

Temporal Distribution ◽

Alluvial Fan ◽

Tree Ring Data ◽

Pine Trees ◽

Spatio Temporal ◽

Flow Activity ◽

Abandoned Channels

Abstract. Hydrogeomorphic processes are a major threat in many parts of the Alps, where they periodically damage infrastructure, disrupt transportation corridors or even cause loss of life. Nonetheless, past torrential activity and the analysis of areas affected during particular events remain often imprecise. It was therefore the purpose of this study to reconstruct spatio-temporal patterns of past debris-flow activity in abandoned channels on the forested cone of the Manival torrent (Massif de la Chartreuse, French Prealps). A Light Detecting and Ranging (LiDAR) generated Digital Elevation Model (DEM) was used to identify five abandoned channels and related depositional forms (lobes, lateral levees) in the proximal alluvial fan of the torrent. A total of 156 Scots pine trees (Pinus sylvestris L.) with clear signs of debris flow events was analyzed and growth disturbances (GD) assessed, such as callus tissue, the onset of compression wood or abrupt growth suppression. In total, 375 GD were identified in the tree-ring samples, pointing to 13 debris-flow events for the period 1931–2008. While debris flows appear to be very common at Manival, they have only rarely propagated outside the main channel over the past 80 years. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of four patterns of debris-flow routing and led to the determination of three preferential breakout locations. Finally, the results of this study demonstrate that the temporal distribution of debris flows did not exhibit significant variations since the beginning of the 20th century.

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Debris flows on forested cones – reconstruction and comparison of frequencies in two catchments in Val Ferret, Switzerland

Natural Hazards and Earth System Science ◽

10.5194/nhess-7-207-2007 ◽

2007 ◽

Vol 7 (2) ◽

pp. 207-218 ◽

Cited By ~ 26

Author(s):

M. Bollschweiler ◽

M. Stoffel

Keyword(s):

Debris Flow ◽

Debris Flows ◽

Historical Knowledge ◽

Study Region ◽

Sampling Height ◽

The Alps ◽

Recurrence Intervals ◽

Study Sites ◽

Flow Activity ◽

Different Characteristics

Abstract. Debris flows represent a major threat to infrastructure in many regions of the Alps. Since systematic acquisition of data on debris-flow events in Switzerland only started after the events of 1987, there is a lack of historical knowledge on earlier debris-flow events for most torrents. It is therefore the aim of this study to reconstruct the debris-flow activity for the Reuse de Saleinaz and the La Fouly torrents in Val Ferret (Valais, Switzerland). In total, 556 increment cores from 278 heavily affected Larix decidua Mill., Picea abies (L.) Karst. and Pinus sylvestris L. trees were sampled. Trees on the cone of Reuse de Saleinaz show an average age of 123 years at sampling height, with the oldest tree aged 325 years. Two periods of intense colonization (the 1850s–1880s and the 1930s–1950s) are observed, probably following high-magnitude events that would have eliminated the former forest stand. Trees on the cone of Torrent de la Fouly indicate an average age of 119 years. As a whole, tree-ring analyses allowed assessment of 333 growth disturbances belonging to 69 debris-flow events. While the frequency for the Reuse de Saleinaz study site comprises 39 events between AD 1743 and 2003, 30 events could be reconstructed at the Torrent de la Fouly for the period 1862–2003. Even though the two study sites evince considerably different characteristics in geology, debris-flow material and catchment morphology, they apparently produce debris flows at similar recurrence intervals. We suppose that, in the study region, the triggering and occurrence of events is transport-limited rather than weathering-limited.

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Debris-flow activity in five adjacent gullies in a limestone mountain range

Geochronometria ◽

10.1515/geochr-2015-0007 ◽

2015 ◽

Vol 42 (1) ◽

Cited By ~ 10

Author(s):

Klaus Schraml ◽

Markus Oismüller ◽

Markus Stoffel ◽

Johannes Hübl ◽

Roland Kaitna

Keyword(s):

Debris Flow ◽

Debris Flows ◽

National Park ◽

Regional Scale ◽

Field Investigation ◽

Mountain Range ◽

Sediment Sources ◽

Aerial Vehicle ◽

Flow Activity ◽

Local Variability

Abstract Debris-flows are infrequent geomorphic phenomena that shape steep valleys and can repre-sent a severe hazard for human settlements and infrastructure. In this study, a debris-flow event chro-nology has been derived at the regional scale within the Gesäuse National Park (Styria, Austria) using dendrogeomorphic techniques. Sediment sources and deposition areas were mapped by combined field investigation and aerial photography using an Unmanned Aerial Vehicle (UAV). Through the analysis of 384 trees, a total of 47 debris-flows occurring in 19 years between AD 1903 and 2008 were identified in five adjacent gullies. Our results highlight the local variability of debris-flow activi-ty as a result of local thunderstorms and the variable availability of sediment sources.

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Periglacial preconditioning of debris flows in the Southern Alps, New Zealand

10.26686/wgtn.17008210 ◽

2021 ◽

Author(s):

◽

Katrin Sattler

Keyword(s):

New Zealand ◽

Debris Flow ◽

Debris Flows ◽

Southern Alps ◽

Ice Growth ◽

Frost Weathering ◽

Flow Formation ◽

Flow Systems ◽

Spatially Distributed ◽

Flow Activity

The lower boundary of alpine permafrost extent is considered to be especially sensitive to climate change. Ice loss within permanently frozen debris and bedrock as a consequence of rising temperature is expected to increase the magnitude and frequency of potentially hazardous mass wasting processes such as debris flows. Previous research in this field has been generally limited by an insufficient understanding of the controls on debris flow formation. A particular area of uncertainty is the role of environmental preconditioning factors in the spatial and temporal distribution of debris flow initiation in high-alpine areas. This thesis aims to contribute by investigating the influence of permafrost and intensive frost weathering on debris flow activity in the New Zealand Southern Alps. By analysing a range of potential factors, this study explores whether debris flow systems subjected to periglacial influence are more active than systems outside of the periglacial domain. A comprehensive debris flow inventory was established for thirteen study areas in the Southern Alps. The inventory comprises 1534 debris flow systems and 404 regolith-supplying contribution areas. Analysis of historical aerial photographs, spanning six decades, identified 240 debris flow events. Frequency ratios and logistic regression models were used to explore the influence of preconditioning factors on the distribution of debris flows as well as their effect on sediment reaccumulation in supply-limited systems. The preconditioning factors considered included slope, aspect, altitude, lithology, Quaternary sediment presence, neo-tectonic uplift rates (as a proxy for bedrock fracturing), permafrost occurrence, and frost-weathering intensity. Topographic and geologic information was available in the form of published datasets or was derived from digital elevation models. The potential extent of contemporary permafrost in the Southern Alps was estimated based on the statistical evaluation of 280 rock glaciers in the Canterbury region. Statistical relationships between permafrost presence, mean annual air temperature, and potential incoming solar radiation were used to calculate the spatially distributed probability of permafrost occurrence. Spatially distributed frost-weathering intensities were estimated by calculating the number of annual freeze-thaw cycles as well as frost-cracking intensities, considering the competing frost-weathering hypotheses of volumetric ice expansion and segregation ice growth. Results suggest that the periglacial influence on debris flow activity is present at high altitudes where intense frost weathering enhances regolith production. Frost-induced debris production appears to be more efficient in sun-avert than sun-facing locations, supporting segregation ice growth as the dominant bedrock-weathering mechanism in alpine environments. No indication was found that permafrost within sediment reservoirs increases slope instability. Similarly, the presence of permanently frozen bedrock within the debris flow contribution areas does not appear to increase regolith production rates and hence debris flow activity. Catchment topography and the availability of unconsolidated Quaternary deposits appeared to be the cardinal non-periglacial controls on debris flow distribution. This thesis contributes towards a better understanding of the controls on debris flow formation by providing empirical evidence in support of the promoting effect of intense frost weathering on debris flow development. It further demonstrates the potential and limitations of debris flow inventories for identifying preconditioning debris flow controls. The informative value of regional-scale datasets was identified as a limitation in this research. Improvement in the spatial parameterisation of potential controls is needed in order to advance understanding of debris flow preconditioning factors.

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The geomorphic impact of large landslides: A case-study of the actively moving Alpine Gardens Landslide, Fox Glacier Valley, West Coast, New Zealand.

10.5194/egusphere-egu2020-12411 ◽

2020 ◽

Author(s):

Saskia de Vilder ◽

Chris Massey ◽

Garth Archibald ◽

Regine Morgenstern

Keyword(s):

New Zealand ◽

Debris Flow ◽

West Coast ◽

Debris Flows ◽

Sediment Flux ◽

Valley Floor ◽

Aerial Imagery ◽

Glacier Valley ◽

Flow Activity

Large landslides can result in significant geomorphic impacts to fluvial systems, via increased sediment input and subsequent changes to channel behaviour. We present a case-study of the actively moving&#160; &#820;65 M m&#179; Alpine Gardens Landslide in the Fox Glacier Valley, West Coast, New Zealand, to analyse the ongoing geomorphic impacts within the valley floor. Debris flows, sourced from the toe of the landslide, travel down Mill&#8217;s Creek and deposit sediment on the debris fan at its confluence with the Fox River. This debris flow activity and associated changes in sediment flux and fluvial behaviour have resulted in re-occurring damage to, and current closure of roads and tracks within the Fox Glacier Valley floor, impacting access to the Westland Tai Poutini National Park, the Fox Glacier, associated tourism, and the Fox Glacier township economy.Initial movement of the Alpine Gardens landslide was detected in 2015, with aerial imagery analysis between March 2017 and June 2018 indicating that the landslide may be accelerating. This acceleration may potentially result in increased debris flow activity within the landslide complex and sediment flux into the Fox River. To monitor and understand the controls on movement rate, we installed a continuous GPS monitoring station along with rainfall gauges on the landslide in February 2019. On average, the landslide moves at a rate of 0.12 m/day &#177; 0.13 m/day, however this rate of movement of the landslide is closely correlated to and fluctuates with rainfall. Significant accelerations of 0.5 m/day have occurred after heavy rainfall, with these rainfall events also resulting in large debris flows.We document and investigate the geomorphic impact of the Alpine Gardens landslide on the Mill&#8217;s Creek debris fan and Fox Glacier Valley floor via terrestrial laser scanning, airborne LiDAR, UAV surveys and aerial imagery. From this, we derive a time-series of nine surface change models to document the sediment flux within the Alpine Gardens Landslide and Mill&#8217;s Creek debris fan complex. Our initial results reveal that between March 2017 and June 2019, approximately 14.7 M m&#179; was eroded from the landslide, of which 3.7 M m&#179; was deposited directly on the debris fan. A further 9.6 M m&#179; has been transported downstream into the fluvial system. Upstream aggradation has also occurred, with 1.1 M m&#179; deposited in the river valley immediately upstream of the debris fan between June 2018 and June 2019. Continued monitoring of the Alpine Gardens Landslide and volumetric changes of the landslide complex allows us to understand the controls on the movement and sediment flux within the landslide and the geomorphic impact of large actively moving landslides on the valley floor, particularly within alpine and glacial environments.&#160;

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Climate change impacts on sediment yield and debris-flow activity at the Illgraben, Switzerland

10.5194/egusphere-egu2020-17017 ◽

2020 ◽

Author(s):

Jacob Hirschberg ◽

Simone Fatichi ◽

Georgie Bennett ◽

Brian McArdell ◽

Stuart Lane ◽

...

Keyword(s):

Climate Change ◽

Debris Flow ◽

Surface Runoff ◽

Debris Flows ◽

Cascade Model ◽

Weather Generator ◽

Climate Scenarios ◽

Climate Services ◽

The Future ◽

Flow Activity

Debris flows are rapid mass movements composed of a mixture of water and sediments and often pose a danger to humans and infrastructure. In the Alpine environment, they are mostly triggered by intense rainfall, snowmelt or a combination thereof, and conditioned by sediment availability. Their occurrence is expected to increase in a warmer climate due to changes in the hydrological regime (e.g. higher rainfall intensity, lower duration of snow cover). Furthermore, sediment production is likely to accelerate due to permafrost thawing and changes in freeze-thaw cycles, resulting in increased sediment availability. For the purpose of climate change impact assessment on sediment yield and debris-flow activity, interactions and feedbacks of climate and the aforementioned processes need to be considered jointly.In the study presented here, we address this challenge by forcing a sediment cascade model (SedCas1) with precipitation and temperature from a stochastic weather generator (AWE-GEN2) producing ensembles of possible climate in the present and for the future. The chosen study site is the Illgraben, a debris-flow prone catchment in the Swiss Alps which currently produces 3-4 debris flows yearly on average. SedCas conceptualizes a geomorphic system in which hillslopes produce and store sediments from landslides and eventually deliver them to the channels. From there, sediments can be mobilized by concentrated surface runoff and transferred out of the catchment in form of bedload, hypreconcentrated flow, or debris flows, depending on the surface runoff magnitude and the sediment availability. AWE-GEN operates at the hourly scale and is trained for the current climate with observed data and for the future climate using the newest climate change projections for Switzerland CH2018 developed by the National Center for Climate Services3.Preliminary results reveal a likely increase in debris-flow occurrence in the Illgraben in the future. Such an increase can be attributed to an extension in the debris-flow seasonal changes in the discharge regime. Furthermore, the number of landslides filling the sediment storage increases because they are affected by a shorter duration of snow cover and thus greater exposure to freeze-thaw weathering. However, projections are subject to large uncertainties, stemming not only from uncertainty in climate scenarios, but also from internal climate variability. Furthermore, the simplified hillslope weathering and debris-flow triggering mechanisms contribute to the overall uncertainty. Nevertheless, the methodology is thought to be transferable to any sediment-cascade-like catchment where dominant processes are driven by climate. Lastly, this work highlights the importance of considering stochasticity in climate and sediment history for projections of magnitudes and frequencies of relative rare events as debris flows. This allows us to explicitly separate climate change signals in geomorphic processes from fluctuations induced by internal natural variability.REFERENCES1 Bennett, G. L., et al. "A probabilistic sediment cascade model of sediment transfer in the Illgraben." Water Resources Research 50.2 (2014): 1225-1244. doi: 10.1002/2013WR0138062 Fatichi, S., et al. "Simulation of future climate scenarios with a weather generator." Advances in Water Resources 34.4 (2011): 448-467. doi: 10.1016/j.advwatres.2010.12.0133 CH2018 - Climate Scenarios for Switzerland. National Centre for Climate Services (2018): doi: 10.18751/Climate/Scenarios/CH2018/1.0

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The Influence of Debris Flow Activity on the Sediment of the Lake Plansee over 3.6 ka (Tyrol, Austria)

10.5194/egusphere-egu2020-21300 ◽

2020 ◽

Author(s):

Carolin Kiefer ◽

Michael Krautblatter ◽

Christoph Mayr ◽

Patrick Oswald ◽

Michael Strasser

Keyword(s):

Debris Flow ◽

Debris Flows ◽

Geophysical Methods ◽

Fold Increase ◽

Delivery Ratio ◽

Sedimentation Rates ◽

Autochthonous Organic Matter ◽

Flow Activity ◽

Debris Cone

Debris flows represent a widespread geomorphological hazard in mountainous regions. Understanding the long-term dynamics of debris flow activity in view of climate change is crucial for the prevention and mitigation of future events. The activity of debris flows is evidently linked to the magnitude of rainstorms. Dietrich & Krautblatter (2017) found an increase in debris flow volumes after 1980 by a factor of 2 compared to the period 1947-1980 and by a factor of 3 compared to the mean Lateglacial/Holocene debris flow volumes by investigating aerial photos of the surroundings of lake Plansee (Reutte, Austria) and estimating debris flow cone volumes with geophysical methods.In this study, the terrestrial observations of increasing debris flow volumes were compared with the subaquatic deposits from the deepest basin of the lake. The debris flow volume within a three-month period on a large debris cone was monitored by Terrestrial Laserscanning (TLS) and the debris flow activity over the last 3 600 years was reconstructed using sediment cores. Four short cores of up to 145 cm depth were recovered in a transect from the shallow subaquatic debris cone area to the deepest basin of the lake. The grain size, density, Magnetic Susceptibility as well as the d13-C, d15N- and C/N-ratios of the sediment were analyzed.The Terrestrial Laserscans revealed a sediment delivery ratio of 30% for the steep debris cone bordering the lake. In the four correlated short cores, 52 debris flow events were differentiated within the last 3 600 years of sedimentation. The proportion of event layers in the cores ranges between 34% and 57% of the total section thickness. The sedimentation rates from a dated core confirm the increase of debris flow activity that was observed with terrestrial methods by Dietrich & Krautblatter (2017). The sedimentation rates show an 11-fold increase after 1930 compared to the rates before 1930 and a 5-fold to 12-fold increase compared to the average Holocene sedimentation rates in lake Plansee. Three types of event deposits were distinguished according to sedimentological criteria: flood-triggered debris flows, earthquake-induced subaquatic suspension flows and mega-events. The TOC/TN ratios of the sediment reveal a permanent influence of terrestrial carbon on the lake sediment and a mixed source of allochthonous and autochthonous organic matter. Large debris flow events can be distinguished from background sediments by increased d13C isotope ratios.The results of this study reveal further scientific proof for the increase of debris flow activity in conjunction with increasing rainstorm activity. Here we show one of the first long-term archives of debris flow activity in the Northern Alps spanning the last 3 600 years and revealing cyclic shifts in debris-flow transport volumes by one order of magnitude.

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Debris-flow deposition zones – an example from the southern slopes of Stara Planina Mountain (village of Anton area)

Review of the Bulgarian Geological Society ◽

10.52215/rev.bgs.2020.81.3.192 ◽

2020 ◽

Vol 81 (3) ◽

pp. 192-194

Author(s):

Zornitsa Dotseva

Keyword(s):

Debris Flow ◽

Debris Flows ◽

Preliminary Analysis ◽

Field Observations ◽

Geological Characteristics ◽

Mountain Village ◽

Raster Analysis ◽

Hazard Assessments ◽

Flow Activity

The analysis of the deposition zones is one of the main steps in the debris flows hazard assessments. For the area located north and northeast of Anton village is known that in the last 100 years there is at least one debris flow event. Field observations, geological characteristics, and raster analysis for prediction of possible sediment accumulations over the fans, related with debris flow activity were performed for preliminary analysis of the debris flows hazard in the area.

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